In guidance systems for missiles and bombs there is a requirement for considerable dynamic range i.e., the range of input signal strength usable without saturation of the guidance circuits, due to the inverse square of the distance (1/R.sup.2) signal strength relationship.
Adequate dynamic range is normally obtained by using logarithmic amplifiers in the processing circuits. These log amplifiers, for the most part pulse video, do not readily lend themselves to implementation in the monolithic technology. They are built with high quality discretes that must be selected to match characteristics such as offset, gain, etc. and made to track over temperature.
Fast rise-time logarithmic video amplifiers use a summation approach to approximate the logarithmic function. As such, they are really pseudologarithmic amplifiers. They are generally implemented by a multistage parallel-summation technique. Each stage of a generalized logarithmic amplifier consists of a linear amplifier, an attenuator, and a logarithmic amplifier (normally a basic video stage), that has logarithmic characteristics over a limited span of the total dynamic range. The linear amplifiers and attenuators are selected to phase in each logarithmic stage sequentially as a function of input intensity. The stages generally provide from 15 to 20 db dynamic range each.
The technique presently used for logarithmic processing of the guidance signals in monopulse receivers for missiles, bombs and guided projectiles utilizes log amps, sample/hold circuits, peak detector circuits or track and hold circuits and difference circuits. The log amps must have adequate bandwidth to process the input pulse, which can be as narrow as a few tens of nanoseconds, and must provide gain matched outputs for dynamic ranges up to 100db. The output of the difference circuit is a signal that is dependent only on the ratio of the inputs. Thus, the range related 1/R.sup.2 dependency is removed, and the output provides angle information only.